Process for manufacturing flat plastic net

ABSTRACT

The invention provides a process and apparatus for manufacturing flat plastic net. Tubular plastic net is made by extruding plastic material through coaxial circular counter-rotating die members to form a net structure having a plurality of crossing strands. The angular displacement of successive longitudinally-aligned junctions of the crossing strands is controlled, preferably by controlling the relative counter-rotation of the extrusion die members, and successive longitudinally aligned junctions are cut longitudinally within each junction. The tubular net so cut is opened out to form flat net. 
     The flat net so produced, the edges of which are characterized by successive longitudinal edge junctions, in which each edge junction is cut longitudinally within the junction, is particularly useful for fencing material, for example.

The present invention relates to a process and apparatus formanufacturing flat plastic net from extruded plastic tubular net.

The term "plastic" used herein refers to synthetic thermoplasticpolymers capable of melt extrusion, natural or synthetic rubberssubsequently vulcanized or containing vulcanizing agents, thosethermosetting plastic materials or mixtures thereof with thermoplasticmaterials which are capable of melt extrusion, and foamable compositionscomprising any of the above materials.

Extrusion of such plastics, through annular dies counter rotatingrelatively to one another, to form a tubular net comprising crossingstrands of plastic is known. A process for extrusion of such a tubularnet is described in Canadian Pat. No. 643 076 which issued June 19, 1962to F. B. Mercer.

Tubular net is used for a variety of purposes, for example, it is usedin the manufacture of hair rollers, or for packaging e.g. Christmastrees, toys, poultry, meats, onions and other comestibles. For somepurposes, however, it is desirable that the net be in a flat form, forexample for use as a fencing material, or as a guard for preventingleaves etc. from falling into open eavestroughs.

It is possible to produce flat net by a process in which a plastic isextruded through linear dies. However, from the vantage points ofeconomy and flexibility of manufacture, it would be advantageous to beable to produce flat net from tubular net, using a tubular extrusionprocess, in which tubular net so formed is slit longitudinally. Severaldisadvantages exist in known methods of manufacturing flat net fromtubular net. One such disadvantage is that at least one edge of the flatnet tends to be rough i.e. some of the strands, of which the net iscomposed, protrude at the edge of the net. Apart from beingaesthetically objectionable, roughness of the edges is particularlyundesirable with net having thick or stiff strands, as for example infencing material, as the protruding strands may tend to snag on clothingand skin, and therefore is regarded as a safety hazard.

One method of producing net with finished edges is disclosed in U.S.Pat. No. 3,089,804 which issued May 14, 1963 to C. D. Gutierrez, inwhich two parallel strands of plastic material are extrudedsimultaneously with and longitudinally onto tubular net, the tubular netsubsequently being cut between the parallel strands. This methodproduced a flat net in which protruding strands at each edge areoverlayed with a strand lying close to and parallel to each edge.

It has now been found that flat net having edges substantially devoid ofprotruding strands may be formed from tubular plastic net, without theneed for overlaying edge strands, by controlling the angulardisplacement of successive longitudinally aligned junctions of crossingstrands of the tubular net about the axis of extrusion relative to afixed plane passing through the axis of extrusion and sometimeshereinafter referred to as the angular displacement of successive netjunctions, and subsequently cutting the net longitudinally within thestrand junctions.

Accordingly the present invention provides a process for manufacturingflat net from tubular plastic net, comprising the steps of

(a) extruding plastic material through coaxial circular counter-rotatingdie members so as to form, about an axis of extrusion, a tubular netstructure comprising a plurality of crossing strands;

(b) controlling the angular displacement of successive substantiallylongitudinally-aligned junctions of the crossed strands about the axisof extrusion and relative to a fixed plane passing through the axis ofextrusion, to minimize said angular displacement;

(c) cutting successive longitudinally-aligned junctions longitudinallyand within each junction, along at least one line of successivelylongitudinally-aligned junctions, such that the tubular net is adaptedto be opened into flat net; and

(d) advancing the net under tension during the extrusion and cuttingsteps.

In a preferred embodiment the angular displacement of successivelongitudinally-aligned junctions relative to the fixed plane through theaxis of extrusion is minimized by controlling the relativecounter-rotation of the extrusion die members.

In another embodiment the cut tubular net is opened out to form flatnet.

In a further embodiment, the plastic material is a polymer of a1-olefin, each strand of said 1-olefin polymer having a weight ofbetween 9 and 30 g/m of length. The plastic material most preferred is apolymer of ethylene or propylene.

The present invention also provides an apparatus comprising incombination

(a) an extruder adapted to extrude, about an axis of extrusion, plasticnet in tubular form through two coaxial circular counter-rotating diemembers, said plastic net comprising a plurality of crossing plasticstrands;

(b) a detector adapted to detect the angular displacement ofsubstantially successive longitudinally-aligned junctions of thecrossing strands about the axis of extrusion and relative to a fixedplane through the axis of extrusion;

(c) a controller adapted to regulate the relative rotation of the diemembers to minimize said angular displacement;

(d) guide means, adapted to align successive longitudinally alignedjunctions of the strands of the tubular net with a cutting means, saidcutting means being adapted to cut each junction longitudinally andwithin each junction; and

(e) advancing means adapted to move the tubular net from the extruderpast said cutting means, and to tension the tubular net during extrusionof the plastic net.

In a preferred embodiment the apparatus includes a means to open the cuttubular net into flat net.

In another embodiment the detecting means comprises a rotatable toothedwheel having each tooth adapted to engage successivelongitudinally-aligned apertures in the net, said rotatable wheel beingadapted to detect the angular displacement of successivelongitudinally-aligned junctions, and the control means comprises acontroller to regulate the speed of rotation of one of the die membersto minimize the angular displacement of successivelylongitudinally-aligned junctions, the speed of rotation of the other diemember remaining substantially constant.

In yet another embodiment the guide means comprises

(a) a rotatable toothed wheel having each tooth adapted to engage withsuccessive longitudinally-aligned apertures in the net, said rotatablewheel being fixedly connected to the cutting means such that the cuttingmeans is aligned with successive longitudinally-aligned junctions and

(b) an engaging means adapted to maintain the tubular net in positiveengagement with the cutting means at the point of cutting.

The present invention further provides a flat plastic net structurecomprising a first plurality of parallel strands crossing and joined toa second plurality of parallel strands at junctions, said first andsecond pluralities of parallel strands being at an angle to alongitudinal edge of the net, said net structure having two longitudinaledges characterized by successive longitudinal edge junctions, each edgejunction being cut longitudinally within said edge junction.

In a preferred embodiment the plastic material is a polymer of a1-olefin, especially a polymer of ethylene or propylene, and moreespecially a copolymer of ethylene and 1-butene.

In a further embodiment each strand of the net structure of the polymerof the 1-olefin has a weight of between 11 and 13 g/m of length, and thenet structure has a weight of between 360 and 720 g/m².

The invention may be illustrated by reference to the accompanyingdrawings wherein:

FIG. 1 is a schematic view of a portion of one edge of the flat netproduced using the process of the present invention.

FIG. 2 represents a schematic view partly in cross-section of apreferred embodiment of an apparatus of the present invention arrangedto carry out a process of the present invention.

FIG. 3 is a schematic representation of a preferred control means of theapparatus of the present invention.

FIG. 4 is a schematic view of a portion of a preferred guiding andcutting means fixedly connected one to the other.

Referring to FIG. 1, the flat net produced by the process of the presentinvention comprises a first plurality of parallel strands 1, crossing asecond plurality of parallel strands 2, joined together at each point ofcrossing at junctions 3. One of the longitudinal edges is defined byedge portions 5 and 6 of strands 1 and 2 respectively terminating atlongitudinally-aligned junctions 7 which have been formed by cuttingjunctions longitudinally within the junction. The pluralities ofparallel strands 1 and 2 are angled relative to one another so that theapertures 4 bounded by adjacent strands of both first and secondpluralities of parallel strands are generally rhombohedral or square inshape, and are also angled relative to the longitudinal direction of theedge.

Referring now to FIG. 2, the apparatus comprises

(a) an extruder 10 having a hopper (not shown) for introducing plasticmaterial to the extruder, rotatable coaxial die members 11 and 13, innerdie member 11 being driven by variable speed motor 15 through driveshaft 12 and outer die member 13 being driven by another motor (notshown) through drive shaft 14;

(b) a circular expansion mandrel 16, the centre of which lies on thesame axis as the centre of the coaxial dies, said axis being sometimesherein referred to as the axis of extrusion;

(c) a means 20 for detecting angular displacement about the axis ofextrusion (shown by broken line A) from a fixed plane passing throughthe axis of extrusion;

(d) a means 50 for controlling the angular displacement oflongitudinally-aligned strand junctions hereinafter described morefully, by manipulating the power source (not shown) of motor 15;

(e) a means 30 for guiding and cutting longitudinally aligned junctionsof crossing strands;

(f) nip rolls 40 and 41, at least one of which is driven by a motor (notshown); and

(g) bar 42 over which flat net is guided before being wound up on aspool or mandrel (not shown). Provision may also be made to cut flat nettransversely, for example, by a guillotine.

As will be described more fully hereinafter, tubular net indicated by 17and 18 is extruded through die lips 11 and 13, tubular net 17 being inan expandable state and tubular net 18 being fully expanded.

Referring now to FIG. 3, the means 20 shown in FIG. 2, for detecting theangular displacement of successive net junctions of the net 18,comprises a rotatable toothed wheel 21 journalled on axle 22 in forks 23of pivot rod 24. The plane through the teeth 25 of the toothed wheel isparallel to the extrusion axis, A (see FIG. 2).

Again referring to FIG. 3, the teeth 25 may be pointed so that theleading and trailing edges of each tooth may engage withlongitudinally-aligned and opposing corners of an aperture, or may beshaped at the point to be substantially the male counterpart of theshape of the aperture.

Pivot rod 24 is pivotted at the end furthest away from the toothedwheel, by bolt 26 which is inserted and held in an immovable frame (notshown) such that toothed wheel 21 may move in a direction substantiallyperpendicular to the plane through the teeth of the toothed wheel. Oneither side of pivot rod 24 are means 27a and 27b for detecting themovement of pivot rod 24 about its pivot point at bolt 26. Eachdetecting means 27a and 27b may be an on/off device, for example amicroswitch or may be combined into a continuous device for detectingposition, for example a mechanical pointer connected to a linearpotentiometer. The type of detector chosen will be governed by theaccuracy of control required which in turn will depend in part upon thewidth of the junctions. By the width of a net junction is meant thedistance across the net junction in the direction of the circumferenceof the tubular net. Detecting means 27a and 27b are capable of sending asuitable signal e.g. an electric or pneumatic signal to controller 50through signal transmission lines 51a, and 51b respectively. Controller50 has a set point and means for comparing signals from transmissionlines 51a and 51b with the set point, and a transmission means 52 fortransmitting a control signal to motor 15. Pivot rod 24 may be supportedby slide 28 attached to the pivot rod and may travel on track 29. Track29 is arcuate, centred about bolt 26 and is attached to an immovableframe (not shown). As may be apparent to those skilled in the art slide28 and track 29 may be replaced by equivalent means for supporting thepivot rod while allowing the pivot rod to move freely above its pivot,at bolt 26.

Referring now to FIGS. 2 and 4 the guiding and cutting means 30comprises a rotatable toothed wheel 31 mounted on an axle 32 journalledin a forked connecting bar 33. Connecting bar 33 is fixedly connected tocutter pivot rod 34 at one end of which is journalled to cutter disc 35,the other end of which is pivotably connected by bolt 36 to an immovableframe (not shown). The plane through the teeth of toothed wheel 31 alsopasses through the plane through the cutting edge of cutter disc 35 andis parallel to the axis of extrusion A. Toothed wheel 31 may be situatedafter or preferably before cutter disc 35. Toothed wheel 31 is freelyrotatable while cutter disc 35 is driven by a motor (not shown). Guideshoe 37 comprises two parallel bars or plates which straddle the teethof toothed wheel and the cutting edge of the cutter disc, the end of thebars nearest the extruder being bend generally towards the axis ofextrusion to form a throat into which part of the tubular net may enter.Guide shoe 37 is adapted to guide the net and hold it in cooperativeengagement with toothed wheel 31 and cutter disc 35.

As was hereinbefore mentioned at least one of nip rolls 40 and 41 isdriven, by a motor (not shown), each nip roll being suitably journalledin bearings. Bar 42 may be fixed, or rotatable, and should be at leastas long as the circumference of the expanded net i.e. the width of theflat net.

A preferred operation of the apparatus described above is describedhereinafter, reference being made to the drawings.

Pellets of plastic material are gravity fed from the hopper into theextruder 10 which is heated. The plastic material, when molten, isextruded between the counter-rotating coaxial die members 11 and 13. Themolten plastic exits from the counter-rotating dies in two pluralitiesof parallel strands 1 and 2, the two pluralities of strands being at anangle to one another. One of the pluralties of parallel strands isoverlayed on the other plurality of parallel strands to form a nethaving a plurality of crossing junctions 3. As the dies are coaxial,tubular net is continuously being formed. Upon emergence of the plasticfrom between the die members, the plastic is still molten and remains ina molten or semi-molten state while overlaying of the strands occur sothat crossing strands fuse at their junctions. The plastic tubular netsoformed is normally expanded while in the formative state over theexpansion mandrel 16 and simultaneously cooled to its solid state bycascading water over and around the expanding net 17.

It is important for the process of the present invention that successivelongitudinally aligned junctions remain essentially parallel to the axisof extrusion, otherwise a bias is introduced which if allowed to developunchecked would make the cutting step of the process difficult and thenet so formed by the process difficult to use in some applications e.g.fencing. The purpose of detecting means 20 and controller 50 is toessentially eliminate development of the bias hereinabove described.

The expanded net 18, after passing over expansion mandrel 16 is pulleddownwards by the pair of nip rolls 40 and 41. The rate of rotation ofthe driven nip rolls is manually adjustable and is set at a speed toensure that the net structure has the desired configuration i.e. shapeof the apertures e.g. square or rhombohedral. Prior to being pulledthrough the nip rolls, a part of the tubular net comes in contact withdetecting means 20 and guiding and cutting means 30. As the tubular netpasses by detecting means 20, successive teeth 25 of toothed wheel 21cooperatively engage with successively longitudinally aligned aperturesin the tubular net. In the event that the angular displacement ofsuccessive junctions of the tubular net deviates from a fixed planethrough the axis of extrusion, the toothed wheel 21 will be caused todeviate about pivot bolt 26. The deviation of toothed wheel 21 aboutpivot bolt 26 is detected by contact of pivot rod 24 with one of thedetecting means 27a or 27b. Contact of the pivot rod 24 with one of thedetecting means, for example 27a, causes a signal to be sent tocontroller 50 via transmission line 51a. The signal, which is indicativeof the position of the pivot rod and therefore the angular displacementof successive net junctions, is compared to a set point, which isequivalent to no angular displacement. On the basis of such comparison asignal is sent via transmission line 52 to motor 15 to speed up or slowdown motor 15, as is dictated by the direction of movement of pivot rod24. Through use of such detection and control means it is possible tocontrol the angular displacement of successive net junctions closely,thereby substantially eliminating the aforementioned bias.

Subsequent to passing detecting means 20, the tubular net is pulled bynip rolls 40 and 41 past guiding and cutting means 30. Successive teethof toothed wheel 31 cooperatively engage with successive longitudinallyaligned apertures in the tubular net 18. The tubular net is pressedagainst toothed wheel 31 by guide shoe 37. The successive longitudinallyaligned junctions of tubular net 18 are thus firmly guided as theytravel past the toothed wheel and are also aligned with the cutting edgeof cutter disc 35 as a result of the relative fixed alignment of thecutter disc 35 with toothed wheel 31. Cutter disc 35, which is rotatedby a motor (not shown), cuts diagonally across the aligned junctions 3of the tubular net 18 to produce cut junctions 7 as shown in FIG. 1.

After being cut by cutting disc 35 the net is pulled through nip rolls40 and 41 as hereinbefore described. After passing through the nip rollsthe cut net is opened out into a substantially flat net 19 and pulled bymeans (not shown) over guide bar 42. Generally it is desirable then towind the flat net into rolls either manually or more conveniently on adriven mandrel. Provision is made to cut the flat net transversely, sothat rolls of the net are of convenient lengths of net and/or ofconvenient weight for further handling.

The following example is intended to illustrate an embodiment of theinvention without limiting the claims.

EXAMPLE

The apparatus was substantially as hereinbefore described and conformedto dimensions and construction as hereinbelow described.

An extruder, available from National-Eyre and equipped withcounter-rotating dies of the kind described in Canadian Pat. No. 871 206to McHardy and Urguhart dated May 18, l971 was used. Expansion mandrel16 was an aluminum ring 38 cm in outside diameter, supported by a shaftfrom the inner die lip 8.75 cm from the lower face of the die members.

Detecting means 20 was about 37.5 cm below the die members. The teeth 25of toothed wheel 21 were triangular in shape and lying in the same planeas the body of the toothed wheel. The diameter measured across thecircumference joining the points of the teeth was 15 cm and the diametermeasured across the circumference joining the notches between the teethwas 10 cm.

The length of pivot rod 24 between axle 22 and the pivot bolt 26 was 35cm. Two microswitches 27a and 27b were placed 10 cm from pivot bolt 26and so placed as to detect deviations of pivot rod 24 of 12.5 mm thusallowing the toothed wheel to hunt over a distance of 12.5 mmtransversely to the axis of extrusion. The outputs of the microswitcheswere connected to a commercially available controller (50). The outputof the controller was used to manipulate the output of a speed controlcontroller for motor 15 which controlled the angular speed of the innerdie member 11.

Toothed wheel 21 was so positioned that the plastic net 18 when passingthe toothed wheel was cooperatively engaged with teeth 25 i.e. bolt 26was about 17.5 cm from the axis of extrusion.

Guiding and cutting means 30 was approximately 1.2 m below the die lips.Guide toothed wheel 31 was of the same dimensions as toothed wheel 21,the axis of rotation beng 30 cm above the axis of rotation of cuttingdisc 35. Cutting disc 35 was 15 cm in diameter, 2 mm thick, and drivenby a 0.125 hp air motor. The distance between axle 32 and bolt 35 was 30cm. The cutting disc and toothed wheel 31 were fixedly connected byforked connecting bar 33 and cutter pivot rod 34, substantially as shownin FIG. 4.

Guide shoe 37 consisted of two bars, each 25 cm long and 12.5 mm indiameter, having the first 7.5 cm of the end nearest the extruder benttowards the extrusion axis at an angle of about 20°. The bars, whichwere attached to the supports for a cutting disc guard assembly (notshown), were 8 cm from the rotational axis of toothed wheel 31 andcutter 35.

Nip roll 40 was 16.25 cm in diameter and 120 cm in length and free torotate about its longitudinal axis. Nip roll 41 was of similardimensions and driven by a 1 hp motor. The nip rolls were positionedapproximately 2 m below the die lips.

Bar 42 was circular in cross section having a diameter of 15 cm andpositioned 2.7 m above the die lips, 3 m laterally from the die lips andparallel to nip rolls 40 and 41.

A guillotine was provided to cut the flat net into suitable lengths anda coreless mandrel was used to wind flat net into coils.

98 Parts of polyethylene pellets available under the trademark SCLAIRfrom Du Pont of Canada Limited were fed into the extruder hopper forevery part of green colour concentrate pellets, available from Dunlop ofCanada Limited. The blend of pellets was melt-extruded by the extruderthrough the die lip members.

The motors controlling the angular speeds of the die lips were set sothat the outer die member rotated at about 5 rpm and the inner membercounter-rotated at about 5.02 rpm relative to the outer die member. Theexpanding net was pulled over the expansion mandrel and cooled withwater at a temperature of about 5° C. The extrusion rate, the die memberrotational speed and the nip roll speeds were adjusted so that theplastic net had substantially square apertures of about 30 mm betweenparallel strands and had a weight of 220 g/m length of tube. The tubularnet was formed at a rate of about 3 meters per minute.

The cutter disc 35 was rotated at an angular speed of about 1800 rpm.

The flat net so formed had a bias of no more than 0.2 cm over a lengthof 15 m and about 97% of the cut junctions were cut completely withinthe junction.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A process formanufacturing flat net from tubular plastic net, comprising the stepsof(a) extruding plastic material through coaxial circularcounter-rotating die members so as to form, about an axis of extrusion,a tubular net structure comprising a plurality of crossing strands; (b)controlling the angular displacement of successive substantiallylongitudinally-aligned junctions of the crossed strands about the axisof extrusion and relative to a fixed plane passing through the axis ofextrusion at a pre-determined value, to minimize said angulardisplacement; (c) cutting successive longitudinally-aligned junctionslongitudinally and within each junction, along at least one line ofsuccessively longitudinally-aligned junctions, such that the tubular netis adapted to be opened into flat net; and (d) advancing the net undertension during the extrusion and cutting steps.
 2. The process accordingto claim 1 wherein the angular displacement of successivelongitudinally-aligned junctions relative to the fixed plane through theaxis of extrusion is minimized by controlling the relativecounter-rotation of the extrusion die members.
 3. The process accordingto claim 1 wherein the cut tubular net is subsequently opened out ofform flat net.
 4. The process according to claim 2 wherein the cuttubular net is subsequently opened out to form flat net.
 5. The processaccording to claim 2 wherein the plastic material is a polymer of a1-olefin.
 6. The process according to claim 5 wherein each strand ofsaid 1-olefin polymer has a weight of between 11 and 13 g/m length. 7.The process according to claim 4, 5 or 6 wherein the plastic material isa plastic material selected from the group consisting of polyethylene orpolypropylene.